The UK prime minister and US president agreed in a phone call the ‘completely unacceptable’ deployment of chemical weapons ‘would force them to revisit their approach so far’.

Mr Obama has previously warned any such deployment by the regime of Syrian president Mr al-Assad would be a ‘red line’ with ‘enormous consequences’.

Western powers have previously ruled out military intervention in Syria’s civil war, but diplomatic efforts to curtail the violence have repeatedly failed.

Russia, Syria’s only remaining major ally, has along with China blocked efforts by the United Nations Security Council to present a united front on stopping the fighting with joint UN-Arab League envoy Kofi Annan eventually resigning to be replaced by Algeria’s Lakhdar Brahimi.

Responding to recent warnings over chemical weapons, Moscow said Western powers were ‘openly instigating’ armed conflict in Syria by encouraging rebels to fight on.

More than 20,000 people are thought to have died in the 17-month conflict, which began as peaceful protests at the start of the Arab Spring but spiralled into a full-blown civil war following a brutal government crackdown.